joelhawksley · Aug 31, 2026
diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
@@ -1,3 +1,16 @@
+*   Reduce allocation overhead of rendering `render_in`-compatible objects.
+
+    PR #50623 introduced a ~5x allocation regression for objects rendered via
+    `render(renderable)` (including ViewComponent) because every such call
+    began flowing through the standard template rendering pipeline. This
+    change memoizes per-class metadata used by `Template::Renderable`,
+    avoids redundant `prepend_formats` writes, reuses the `TemplateRenderer`
+    within a `Renderer`, gates the `render_template.action_view`
+    notification payload construction on active subscribers, and reuses
+    a pooled `Template::Renderable` when no block is supplied.
+
+    *Joel Hawksley*
+
 *   Allow `translate`'s (and `t`'s) `scope:` option to be resolved relative to
     the current template when it starts with a period, mirroring the existing
     behavior for the key argument.

diff --git a/actionview/lib/action_view/renderer/abstract_renderer.rb b/actionview/lib/action_view/renderer/abstract_renderer.rb
@@ -172,7 +172,10 @@ def prepend_formats(formats) # :doc:
         formats = Array(formats)
         return if formats.empty? || @lookup_context.html_fallback_for_js
 
-        @lookup_context.formats = formats | @lookup_context.formats
+        current = @lookup_context.formats
+        return if (formats - current).empty?
+
+        @lookup_context.formats = formats | current
       end
 
       def build_rendered_template(content, template)

diff --git a/actionview/lib/action_view/renderer/renderer.rb b/actionview/lib/action_view/renderer/renderer.rb
@@ -55,7 +55,7 @@ def cache_hits # :nodoc:
 
     private
       def render_template_to_object(context, options, &block)
-        TemplateRenderer.new(@lookup_context).render(context, options, &block)
+        (@_template_renderer ||= TemplateRenderer.new(@lookup_context)).render(context, options, &block)
       end
 
       def render_partial_to_object(context, options, &block)

diff --git a/actionview/lib/action_view/renderer/template_renderer.rb b/actionview/lib/action_view/renderer/template_renderer.rb
@@ -2,6 +2,9 @@
 
 module ActionView
   class TemplateRenderer < AbstractRenderer # :nodoc:
+    RENDER_TEMPLATE_NOTIFICATION = "render_template.action_view"
+    private_constant :RENDER_TEMPLATE_NOTIFICATION
+
     def render(context, options, &block)
       @details = extract_details(options)
       template = determine_template(options, &block)
@@ -41,7 +44,14 @@ def determine_template(options, &block)
           end
           Template::Inline.new(options[:inline], "inline template", handler, locals: keys, format: format)
         elsif options.key?(:renderable)
-          Template::Renderable.new(options[:renderable], &block)
+          if block.nil?
+            pooled = (@_pooled_renderable ||= Template::Renderable.allocate)
+            pooled.instance_variable_set(:@renderable, options[:renderable])
+            pooled.instance_variable_set(:@block, nil)
+            pooled
+          else
+            Template::Renderable.new(options[:renderable], &block)
+          end
         elsif options.key?(:template)
           if options[:template].respond_to?(:render)
             options[:template]
@@ -57,12 +67,16 @@ def determine_template(options, &block)
       # supplied as well.
       def render_template(view, template, layout_name, locals)
         render_with_layout(view, template, layout_name, locals) do |layout|
-          ActiveSupport::Notifications.instrument(
-            "render_template.action_view",
-            identifier: template.identifier,
-            layout: layout && layout.virtual_path,
-            locals: locals
-          ) do
+          if ActiveSupport::Notifications.notifier.listening?(RENDER_TEMPLATE_NOTIFICATION)
+            ActiveSupport::Notifications.instrument(
+              RENDER_TEMPLATE_NOTIFICATION,
+              identifier: template.identifier,
+              layout: layout && layout.virtual_path,
+              locals: locals
+            ) do
+              template.render(view, locals) { |*name| view._layout_for(*name) }
+            end
+          else
             template.render(view, locals) { |*name| view._layout_for(*name) }
           end
         end

diff --git a/actionview/lib/action_view/template/renderable.rb b/actionview/lib/action_view/template/renderable.rb
@@ -1,9 +1,15 @@
 # frozen_string_literal: true
 
+require "concurrent/map"
+
 module ActionView
   class Template
     # = Action View Renderable Template for objects that respond to #render_in
     class Renderable # :nodoc:
+      RENDER_IN_ARITY = Concurrent::Map.new
+      FORMAT_RESPONDER = Concurrent::Map.new
+      private_constant :RENDER_IN_ARITY, :FORMAT_RESPONDER
+
       def initialize(renderable, &block)
         @renderable = renderable
         @block = block
@@ -14,9 +20,16 @@ def identifier
       end
 
       def render(context, locals)
-        render_in_method = Kernel.instance_method(:method).bind_call(@renderable, :render_in)
+        klass = @renderable.class
+        arity = if klass.method_defined?(:render_in)
+          RENDER_IN_ARITY.fetch_or_store(klass) do
+            klass.instance_method(:render_in).arity
+          end
+        else
+          Kernel.instance_method(:method).bind_call(@renderable, :render_in).arity
+        end
 
-        if render_in_method.arity == 1
+        if arity == 1
           ActionView.deprecator.warn <<~WARN
             Action View support for #render_in without options is deprecated.
 
@@ -36,7 +49,13 @@ def render(context, locals)
       end
 
       def format
-        @renderable.try(:format)
+        klass = @renderable.class
+        responds = if klass.method_defined?(:format)
+          FORMAT_RESPONDER.fetch_or_store(klass) { true }
+        else
+          @renderable.respond_to?(:format)
+        end
+        responds ? @renderable.format : nil
       end
     end
   end